1. Field of Invention
The present invention relates to systems for optical communication that utilize low power requirements and have a low error rate. In particular, the present invention is directed to methods of using sets of photons with orthogonal properties and their associated quantum states to achieve such communications. In specific embodiments, the invention allows for the use of two photons per bit and time correlation of the same to provide improved noise immunity.
2. Description of Related Art
In the field of free-space communications there exists a need for the transmission and receipt of information over a noisy ambient or background noise source using ultra low power. Whether the communication system uses radio waves (having centemeter to meter wavelengths) or photons (having nanometer to micrometer wavelengths), the fundamental limitation of signal-to-noise ratio (SNR) versus a given, required data transmission rate arises when radiated power is limited and background noise sources are high.
In the area of communications between micro-devices, such as micro-robots or micro-transceivers that have power budgets on the order of microWatts or nanoWatts, the radiated power cannot exceed a given power budget. In order to increase the data transmission rate or throughput, a way of increasing the SNR without increasing the required radiated power is required. In addition, micro-devices also have another primary limitation in that the physical size of the transceiver must be very small. The size of these micro-devices can be on the order of ten square micrometers to one square millimeter.
In order for a data transmitter to be useful for micro-devices, it must be small, i.e., on the order of tens of micrometers to 1 mm, and require very little power (less than microWatts). Furthermore, it must enable a reliable data transmission rate of at least 1 kHz over a distance of at least 100 m, for specific applications. Because of the small size of the devices, it is very difficult to communicate at Radio Frequency wavelengths. This is because the conversion efficiency of electrical energy to radiated electromagnetic energy drops significantly when the RF radiating device is smaller than one quarter of the wavelength of the radio waves, which is the case for micro to millimeter devices. This implies that the portion of the electromagnetic spectrum that should be used for efficient energy conversion is in the optical region. Prior art systems include that optical communication include quantum communication systems that generate photons that share a quantum state, or in other words are entangled.
It also includes optical communication systems based on lasers and single frequency transmissions. Very small communication devices discussed in the prior art are, in general, devices that transmit information at radio frequencies (usual frequencies: 10 KHz to 10 MHz). Previous systems that utilize entangled photon transmission are complicated large-scale systems requiring a ultraviolet (UV) pump laser to excite a non-linear crystal to produce time-coincident photons. Prior art optical communication systems employing photons for both free-space and fiber optic Quantum Key communications have been demonstrated. These systems typically utilize a bright, higher power, synchronization laser in conjunction with a low power photon source. See, for example, “Present and Future Free-Space Quantum Key Distribution,” Nordholt et Al., SPIE Conference: Free-Space LASER Communications Technologies XIV, 20-25 JAN 2002, San Jose, Calif. Such systems are not, however, applicable to the low power and small size devices discussed herein.
As such, there is a need for a system that is physically small and able to provide communications at ultra low power levels. There is also a need for systems that provide communications at sufficiently high SNR that can operate with constrained power budgets.